Conventional laser, or optical, transmitters include a closed-loop compensation circuit for adjusting the current flow across the laser diode. The current flow is continuously adjusted in response to any current level fluctuations in order to maintain a predetermined output optical power level. It is known that surrounding temperature changes are typically the cause of current fluctuations. It will be appreciated that a compensation, or bias, circuit is necessary to maintain the predetermined optical power level over the operating temperature range of the laser diode. A typical temperature range at the laser location in the transmitter is, for example, from −20° C. to +85° C.
Since the conventional optical transmitter operates in a continuous mode, i.e., provides a continuous output optical signal, a closed-loop control or an automatic power control circuit is widely used to control the current fluctuations. More specifically, the closed-loop control circuit continuously monitors and adjusts the current across the diode in order to maintain the desired optical power. FIG. 1 illustrates a simplistic diagram of a closed-loop compensation circuit 100 that is suitable for use in the conventional optical transmitter. A laser diode 105 launches a certain desired level of optical power and a photodiode 110 generates an electrical current that is directly proportional to the optical power. If the power level drifts from its established desired nominal level, a bias control circuit 115 detects the change in the electrical current provided by the photodiode 110 and subsequently varies the laser current until the desired optical power level is again reached. Accordingly, the closed-loop circuit provides constant adjustments to the optical power.
While the closed-loop compensation circuit 100 is appropriate for the conventional optical transmitter, it does not work effectively for a burst-mode laser transmitter. It will be appreciated that the burst-mode transmitter is essentially turned off and does not transmit an optical signal until a burst-mode incoming signal is received. Only upon receiving the incoming signal will the burst-mode transmitter operate in comparison to the constant transmission of optical signals at the output of the conventional transmitters. Accordingly, the closed-loop compensation circuit 100 does not adjust the power level quickly enough to accommodate the burst-mode incoming signals. What is needed, therefore, is a compensation circuit that maintains the desired operating power level in response to any temperature fluctuations within the burst-mode optical transmitter.